Issue link: https://geospatial.trimble.com/en/resources/i/1415420
august 2019 csengineermag.com 13 It's an acknowledged fact that construction practices are changing at a rapid pace. The American Concrete Institute's recently released ACI 318-19: "Building Code Requirements for Structural Concrete" responds to many of these changes. With a focus on safety, economy and sustainability, it addresses many of the new technologies, products and methods that have become common in recent years. Seismic Design New provisions amplify code-prescribed wall design shears based on considerations of wall flexural overstrength and the effects of higher dynamic response modes – the result, in many instances, is a sub- stantial increase in design shears for some walls. The amplified shear forces will better reflect the higher shear forces that have been seen in buildings designed by nonlinear dynamic analysis methods. Based upon laboratory tests as well as observed behavior of structural walls during seismic events, other changes have been incorporated into the code. For example, to improve concrete confinement and lon- gitudinal bar support, ACI 318-19 limits the aspect ratio of hoops in boundary elements and requires that all crossties have seismic hooks at both ends. To avoid brittle fracture of under-reinforced walls, the code also requires some walls to satisfy minimum longitudinal reinforce- ment requirements. New ACI standards (the ACI 550 series) reflect changes made to the 2016 edition of ASCE 7, which incorporated new provisions that called for significant increases in seismic design forces for precast diaphragms. It established new requirements for the design and de- tailing of precast concrete diaphragms, particularly the connections between precast elements. The ACI 550 documents and standards are referenced in 318-19. Deep Foundations ACI 318-19 attempts to eliminate conflicting information in ACI 318, ASCE 7 and IBC regarding design of deep foundations for earthquake- resistant structures, with a goal of having all pertinent concrete-related seismic design and detailing provisions for deep foundations contained in ACI 318. Among other changes, new provisions on precast concrete piles were incorporated; axial load limitations for deep foundations are now included; and strength design requirements were added that are con- sistent with ACI 543. This code change will allow designers of deep foundation members to use either the traditional allowable stresses that have been in the general building codes for years or the strength design method using strength reduction factors that is consistent with the rest of ACI 318-19. Beam Column Joints ACI 318-19 incorporates design provisions for shear strength and reinforcement detailing of beam-column joints of seismic design category (SDC) A frames, ordinary moment frames, intermediate mo- ment frames, and frames that are not part of the seismic force-resisting system in SDC B, C, D, E, and F. Also, existing design provisions for special moment frames were expanded to include shear strength of roof joints. Chapter 15 now requires consideration of the presence of opening and closing moments in corner joints. This includes moment transfer across a diagonal section through a corner joint, which is particularly critical where the joint connects a cantilever member for which no redistribu- tion of moments is possible. Design Verification Using Nonlinear Response History Analysis The use of nonlinear dynamic analysis methods for design of earth- quake-resistant buildings has been increasing recently, especially for design of high-rise buildings. To respond to this change in design practice, ACI 318-19 includes provisions for application of these methods to concrete buildings. The provisions are intended to be fully compatible with Chapter 16 of ASCE 7-16, which contains require- What's New in ACI 318-19: Building Code Requirements for Structural Concrete By Jack P. Moehle, PhD, PE ACI 318-19 includes new and updated code provisions on one-way shear, two-way shear, shear wall drift capacity, seismic design, shotcrete, deep foundations, post- tensioning, precast, durability, lightweight concrete, and more.